Avalanche photodiodes (APDs) are widely employed in high-speed communication systems. For optical receiver applications, for instance, APDs are preferred over many other candidate photodetectors, including PIN diodes, particularly due to their high internal gain characteristics and improved signal-to-noise ratio.
Generally, APDs utilize a region of semiconductor material for receiving incident photons within an appropriate spectral range (for example, 1.2 xcexcm less than xcex less than 1.6 xcexcm), and generating charge carriers in response to the incident light. The generated charge carriers are accelerated towards opposite electrodes by a large applied reverse-bias. At least some of the charge carriers are accelerated into a second region of a high-field semiconductor material where the carriers gain sufficient energy to initiate impact ionization, and hence avalanche multiplication.
One typical APD structure is shown in FIG. 1. This planar, multilayer structure comprises an absorption layer 302, for receiving incident light and creating electrons and holes through the photogeneration process, and a multiplication layer 301, where the avalanche multiplication occurs. These devices may be formed on a semiconductor substrate 300 (such as an n+-type InP wafer). In this example, the absorption layer 302 (e.g. undoped InGaAs) is formed directly above the substrate, followed by, in succession, one or more grading layers 306 (e.g. InGaAs) for grading the band-gap between the absorption and multiplication regions, a thin dose layer 305 (e.g. xe2x88x921000 Angstrom-thick n+-type InP) doped to a very specific areal dose, and the multiplication layer 301 (e.g. undoped InP).
Commonly, a p-type dopant (e.g. Zinc) is added into the structure through diffusion or ion-implantation techniques to form a p-n junction near the surface of the device. As shown in FIG. 1, the thickness of the multiplication layer 301 is determined by the depth to which the p-type dopant diffuses into the device.
Using conventional diffusion or ion implantation techniques, the depth of the diffusion region 303, and thus the thickness of the multiplication layer 301, can be controlled with a generally acceptable degree of accuracy. Typically, the thickness of a multiplication layer 301 defined through diffusion can be controlled to within ∀ 500 Angstoms. This is a satisfactory degree of control for APDs with a sufficiently thick multiplication layer (i.e.  greater than 4 xcexcm). However, this technique may not be accurate enough to produce reliable, high-performance APDs with very thin multiplication layers.
Also shown in FIG. 1 is guard ring 304, which is a second diffused region of a p-type dopant that surrounds the periphery of the diffusion region 303. In an APD, ideally the region of highest gain should be the central, optically active region of the device (i.e. at the planar interface between the diffusion region and the multiplication layer). However, due to the curvature at the periphery of the diffusion region 303, the highest fields are often produced at the edges, rather than the center of the optically active region. The guard ring helps to suppress undesirable edge-breakdown by effectively eliminating the curved periphery of the diffusion region. Although, with the addition of the guard ring, there is a tradeoff in that the overall area of the device, and thus its capacitance, is increased.
Also, because of the relatively large reverse bias (xe2x88x9225V) applied across the p- and n-electrodes, there is typically a large dark current produced in the conventional APD, which negatively effects performance and can shorten the useful lifetime of the device.
An avalanche photodiode (APD) which comprises a semiconductor substrate, an undoped multiplication layer above the substrate, a thin dose layer over the multiplication layer, and an undoped absorbing layer located above the multiplication and dose layers. The APD can additionally comprise p- and n-electrodes for applying a reverse bias across the layers of the device. In operation, incident photons are received in the absorbing layer, which generates charge carriers. At least some charge carriers are accelerated into the multiplication layer, where they gain enough energy to initiate impact ionization, and thus avalanche multiplication.
The multiplication layer is defined through a conventional epitaxial growth process, such as metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). The thickness of the multiplication layer is preferably determined by a relatively controllable and well-understood growth process, rather than by diffusion or ion implantation techniques. The thickness of the multiplication layer is thus controllable to within about ∀ 50 Angstroms. APDs with thin multiplication layers may be reliably produced, including layers less than 4 xcexcm thick, such as a 2 xcexcm or 1 xcexcm-thick multiplication layer.
By designing the APD to have a particularly thin multiplication layer, the statistical distribution of carrier gains may be advantageously narrowed. An APD characterized by low noise and high gain bandwith (GBW) product can thus be produced.
The multiplication layer can comprise any suitable semiconductor material, preferably lattice matched to the APD substrate. The dose layer can comprise this same material that is doped to a very specific areal dose. According to one embodiment, the multiplication layer and dose layer comprise InAlAs.
In one exemplary embodiment, the APD of the present invention comprises a multilayer structure, having, in succession, an n-type substrate, an n-type buffer layer, an n-type field stop layer, an undoped multiplication layer, a p-type dose layer, one or more grading layers, an undoped absorption layer, one or more grading layers, an undoped passivation layer, and a p-type contact layer. The p-side electrode forms an electrical connection with the contact layer, and the n-side electrode forms an electrical connection with either the substrate or buffer layer.
According to one aspect, the second set of grading layers (i.e. between the absorption layer and the passivation layer) is eliminated, and a p-type dopant, such as zinc, is diffused through the surface of the device and at least partially into the absorption layer.
According to another aspect, a three-terminal photodiode device comprises a semiconductor structure having an active region of semiconductor material that receives incident light and generates charge carriers in response to the light. The device also includes a contact layer of a semiconductor material located above the active region, the contact layer comprising a central region and a peripheral region, where the peripheral region is separate from, and concentrically surrounds the central region. The device further includes two electrodes for applying a voltage across the active region, one of these electrodes being in electrical connection with the central region of the contact layer, and a third electrode that is in electrical contact with the peripheral region of the contact layer.
In an APD, for instance, the third electrode comprises a drain-ring disposed on the periphery of the p-side electrode. The drain ring is located outside of the optically active region of the APD. This third electrode can collect charge carriers generated outside of the optically active region of the device.
The present invention also relates to a three-terminal p-type-intrinsic-n-type (PIN) diode having a contact layer comprised of a central region and a peripheral region. The device additionally includes two terminals (electrodes), one of which is electrically connected to the central region of the contact layer, for applying a voltage across the active regions of the device, and a third terminal that is electrically connected to the peripheral region of the contact layer.
The three-terminal structure enables at least some of the Adark current@ generated in the photodiode to be drained off by the third terminal, resulting in a less noisy device. When properly biased, this peripheral terminal can help to manage the dark current over the entire life of the device.
A method for fabricating an APD includes providing a semiconductor substrate, forming an undoped multiplication layer above the substrate, forming a thin dose layer over the multiplication layer, and forming an undoped absorbing layer above the multiplication and dose layers. Each of the multiplication layer, dose layer, and absorbing layer can be fabricated using a known epitaxial growth process, such as metalorganic chemical vapor deposition, or molecular beam epitaxy. In certain embodiments, the multiplication layer is grown to a substantially uniform thickness of approximately 4 xcexcm or less. The thickness of the multiplication layer can also be controlled to within approximately 50 Angstroms.
A method for fabricating a three-terminal photodiode includes providing a semiconductor structure having an active region of semiconductor material that receives incident light and generates charge carriers in response to the light, and forming a contact layer of a semiconductor material above the active region, the contact layer comprising a central region and a peripheral region. The peripheral region is separate from, and concentrically surrounds the central region. A pair of electrodes can also be provided for applying a voltage across the active region of the device, where one of the electrodes is electrically connected to the central region of the contact layer. A third electrode can also be provided in electrical contact with the peripheral region of the contact layer.